the wheels are after-market Momo Thunder alloys in 18". i cant remember what the offset is either, all i know is that they came off a damage free 156 GTA.
I've tried hunting the internet for dimensions of these wheels, but can't find a darn thing.
I've also fitted 3mm Spacer shims on the hub, just so the inside of the tyre clears the suspension strut with plenty of room.
as for the "less tyre on the road", i don't personally think that 10mm over the width of the tyre is going to make a "life or death" change to the handling. I'm not one for booting it around town, and if anything i'm planning up to 2 track days next year, other than that, it may get the odd b-road run with the local AO club. And, as i've mentioned, i have seen stretched tyres before, and i've seen them on a track, it actually stiffens the sidewall of the tyre allowing less flex. In some cases it's a bad idea, in others it can be fairly reasonable. Also, why would these tyre dealers stock these tyre sizes if they were such a bad idea?? because i know for a fact the size i'm after is NOT a standard size.
Like i said, i might pop into the local tyre fitters and see what they say. If they say it's a really bad idea, then i'll forget it.